1. Field of the Invention
The invention relates to the field of color television cameras and more particularly to a contour correction devices required in such cameras.
The separating power of color picture taking tubes is limited by the thickness of the scanning spectrum. The result is a loss of resolution as a function of the frequency, which corresponds to an attenuation of the contours of the image. Moreover, the resolution in the vertical direction is limited by the number of scanning lines.
The purpose of contour correction devices is to compensate for such degradation, resulting in a limitation of the modulation rates, while enhancing the visual appearance of the image without increasing the noise thereof. an efficient contour correction is the result of a compromise between sufficient accentuation of the transitions in the vertical direction and in the horizontal direction of the image and a limitation of the noise, in particular in the uniform color areas.
2. Description of the Prior Art
Several correction systems have been brought into use up to present. The first system, performing well and at the same time representing a fairly simple system, consists in extracting the contour signal, that is to say the detail of the images, from the luminance signal. This method is applied in present coding systems where the pass band of the luminance channel is about four times greater than the pass band of the chrominance channels.
In fact, the first color television cameras were provided with four tubes supplying respectively the luminance Y and chrominance R, G, B signals. Contour corrections from the luminance signals, had the advantage of not being affected by convergence defects of the three color tubes, the modulation rate faithfully following the luminance law.
In the new cameras whose scanning system comprises three tubes, the luminance signal is formed from the green channel, and the details extracted from this channel are nonexistant for a considerable area of the triangle of the colors comprising the whole of the area close to red.
The conventional contour correction system consists in forming an image which is not a sharp image by means of a system of filters from the initial image. This processing is generally applied to the luminance channel alone or to the green channel alone. The differences between the initial image and the filtered image are then calculated to generate and a contour signal. This signal then has the noise removed and is shaped for generating a useful correction signal which is applied simultaneously:
to the filtered image for reconstituting the initial image with the noise removed, and
to the reconstituted image for accentuating the contours after the gamma correction system so as to avoid disturbing this latter.
This correction signal is applied to each of the signals forming the video signal. An example of this type of correction is described in the U.S. Pat. No. 3,732,360. The results obtained with such a system are fairly satisfactory but are however limited by two major drawbacks:
the red and blue channels from which the noise has not been removed, corrected by the contour signals extracted from the green channel, increase the noise in the luminance signal;
the red and blue colors of some images with a low green level are without contours.
To overcome this disadvantage, it has been proposed, for example, in the application GB No. 2 126 828 to extract the contour signal from a luminance signal formed by a combination of signals. The combination coefficients, determined by experience, may be for a combination of the red channel and the green channel: 0.3 for the red channel and 0.7 for the green channel, and the combination gives 0.3 R+0.7 V. Thus, a balanced contour correction is obtained, representative of the luminance, in which a single channel for processing the contour is used, the resultant contour signal being applied to the different channels. Such a system has the advantage of being simple and economical but has a drawback: correction in the red adds contours to the black and white details and noise to the luminance. In addition, any lack of convergence between the green channel and the red channel gives rise to a visible splitting of the contour. Finally, such a solution does not allow the red contour and the green contour to be separately adjusted.
Another solution has been proposed in which the contour signal is formed from the maximum of two color signals, 0.3 R and G. This is obtained from the difference between the signals coming from the two non additive lattices processing respectively the net video signals and the filtered video signals, and extracting the maximum of the net or filtered 0.3 R and G signals. As a rule, this system causes red contours to appear if 0.3 R is greater then G and green contours if G is greater than 0.3 R. The coefficients have been defined as a function of representative cases of a color image, white detail on a black background of black detail on a white background. The coefficient 0.3 is the result of a compromise between a sufficient contour amplitude and limitation of the reduction of the green channel in black-red transitions. This system is also simple and economic like the above described system since it uses a single processing channel for the red and green contours but, also like the preceding system, it is impossible to separately adjust the red contour and the green contour. In addition, this system introduces a considerable reduction of contours in the white-red transitions.